This invention is related to techniques for evaluating the hydrocarbon content and composition of an underground formation. More particularly, the invention offers a method for determining from samples such as drill cuttings whether the asphaltene content of a hydrocarbon formation is high enough to cause the formation to be non-producible.
Because of the difficulties and cost involved in producing highly viscous oils, it is frequently desirable to know the producibility of a hydrocarbon formation without placing a well on test. This is particularly true in geographical areas such as California which have a relatively high percentage of viscous hydrocarbons. Current logging techniques are often ineffective in evaluating the producibility of an underground formation prior to placing a well on test.
In some situations, electric logs do not effectively differentiate between hydrocarbons and water, and rarely between non-producible and producible oil. Even though electric logs may correctly identify the presence of pore-filling carbonaceous material, they are unaffected by the viscosity of the oil which is a significant factor in determining whether oil will flow into the wellbore.
Mud log data is commonly used instead of wireline log data in an attempt to evaluate a formation prior to an expensive DST (Drill Stem Test) or putting a well on production test. The mud logs leave a great deal to be desired as an accurate indicator of the presence of petroleum, and if present, of productive formations.
Fluorescence has been used as a logging technique for detecting oil in drill cuttings for decades. However, the method used to determine the fluorescence of samples at a rig site is a crude method which has not improved appreciably and is severely limited in its usefulness and applicability. At present, fluorescence is determined when an operator shines a broad spectrum ultraviolet light source on cuttings in the hope of seeing substantial fluorescence to indicate the presence of oil. See U.S. Pat. Nos. 2,311,151; 2,337,465; 2,459,512; 2,951,940 and Re. No. 22,081.
There are several inherent problems in current fluorescence logging which make it nonquantitative at best and misleading at worst. First, the excitation source is not concentrated in the spectral region where the oil is most likely to absorb radiation and re-emit that radiation as fluorescence. Second, the oil is quite likely to emit fluorescence at wavelengths predominantly, if not totally, unseen by the human eye. Third, the fluorescence observed by the operator is influenced by the presence of fluorescent minerals such as fluorite. Fourth, the presence or amount of oil on the surface of the cuttings samples may not be representative of the oil in the pore structure of the formation. The mud logger sees only the surface of the samples with this technique. Fifth, operators' description of such fluorescence phenomena is highly subjective. Such commonly used words as strong, weak, bright, dull, yellow, and gold prohibit any quantitative analysis of the data.
U.S. Pat. No. 4,696,903 discloses shining UV light on formation samples and visually noting the color of the fluorescence as well as taking video pictures of the fluorescence for later study. U.S. Pat. No. 4,248,599 discloses a process for determining the API gravity of oil by the use of a flame ionization detector. In this method, the volatile and pyrolyzable components of oil are vaporized. A measurement is made of the ratio of the amount of hydrocarbon vapor produced at temperatures within a selected high temperature range to the total amount of vapor produced. A ratio of fluorescence measured under two conditions is taken in conjunction with the use of the flame ionization detector.
The emission fluorescence of crude oil samples has been studied and recorded over various wavelengths, including ultraviolet wavelengths below 400 nm. Studies which have taken place at the Bartlesville Energy Technology Center have been basically "fingerprint" studies wherein the emission fluorescence of various types of crude oils has been recorded at different excitation wavelengths. This Department of Energy research was a spin-off from earlier efforts by the Bureau of Mines to try to identify crude oil by emission fluorescence for purposes of pollution control. Please see Chisholm, B. R., Eldering, H. G., Giering, L. P., and Hornig, A. W., Total Luminescence Contour Spectra of Six Topped Crude Oils, BETC/RI-76/16, a paper prepared for ERDA for the Bartlesville Energy Research Center in Bartlesville, Okla., November 1976; and Brownrigg, J. T. and Hornig, A. W., Low Temperature Total Luminescence Contour Spectra of Six Topped Crude Oils and their Vacuum Distillate and Residuum Fractions, BETC/RI-78/13, a paper prepared for DOE for the Bartlesville Energy Technology Center, Bartlesville, Okla., July 1978. Similar, non-published fingerprinting work of crude oils by total luminescence spectra has also been performed in unpublished work at Texas A. & M. University.
There is one recently developed process which employs fluorescent measurement to test for the presence of hydrocarbons within drill cuttings. But this process does not give an indication of viscosity or producibility. Further, U.S. Pat. No. 4,609,821 is applicable only to oil base mud drill cuttings. The cuttings are excited with a wide range of UV wavelengths and the emitted radiation is recorded over a wide range of wavelengths to produce an analytical chemical profile. This profile of intensity over multiple wavelengths of excitation and emission radiation is compared with previous profiles to determine the presence of hydrocarbons not associated with the oil base mud.
Molecular fluorescence is discussed in general in Skoog, Douglas, Principles of Instrumental Analysis, Sanders College Publishing, Philadelphia (3rd ed. 1985), pp. 225-240. The reference indicates that the greatest fluorescence behavior occurs with compounds containing aromatic functional groups and offers a table which indicates the UV fluorescence wavelengths associated with numerous benzene derivatives in ethanol solution. Several analytical profiles of hydrocarbons are disclosed wherein fluorescence intensity is plotted over multiple excitation and emission wavelengths.